Leviathan is a double-sided figure, representing the enemy of god, rampant elementalism, and (according to Hebraic text) the free innocence of being animal. Altogether, Leviathan might be seen as nature itself, unstoppable, all-encompassing, frightening in its power and eternal, yet carefree and happy as it is not hampered by the petty problems of man.
Leviathan as a mythological figure is extraordinarily well suited to use as a Satanic archetype, as all the different and seemingly incompatable sides are brought together, made compatable, and are fundamentally compatable with Satanic philosophy.
Incidentally...it is odd that you name yourself after Jormungander and "XIII" as I have named myself after Leviathan and the same number, and we now discuss Leviathan.
Also, you must make your introductory post. I do not see one, and it is required (and well overdue). You ought to post one before our leniency wears thin...
"The devil I'll bring you," answered Hagen. "I have enough to carry with my shield and breastplate; my helm is bright, the sword is in my hand, therefore I bring you naught."